Testing an upstream path of a cable network

ABSTRACT

An apparatus and method for testing an upstream path of a cable network are disclosed. The upstream path is tested by capturing and analyzing upstream data packets generated by a specific terminal device. A test instrument is connected at a node of the cable network. The test instrument establishes a communication session with the headend, informing the headend of an identifier of the device that will generate the test upstream data packet. The test upstream data packet is captured and analyzed at the headend, so that the results of the analysis can be communicated back to the test instrument. To speed up the packet capturing and filtering process, the upstream data packets can be pre-filtered based on packet duration and/or arrival time.

TECHNICAL FIELD

The present invention relates to network testing, and in particular totesting of an upstream signal path of a cable network.

BACKGROUND OF THE INVENTION

Cable networks have, in recent years, moved beyond merely broadcastingtelevision signals over a co-ax cable to subscribers in their homes.Subscribers of a cable network nowadays have a modem allowing thetransmission of digital signals upstream toward the headend of thenetwork. Among many services afforded by cable modems are: an Internetservice, a home shopping service using a television catalogue, and avoice-over-IP phone service.

In bidirectional cable networks, the upstream and the downstream signalsoccupy separate frequency bands called upstream and downstream spectralbands. In the United States, the upstream spectral band typically spansfrom 5 MHz to 42 MHz, while the downstream spectral band typically spansfrom 50 MHz to 860 MHz. Downstream information channel signalsco-propagate in the downstream spectral band, and upstream signalsco-propagate in the upstream spectral band. The frequency separation ofthe upstream and the downstream signals allows bidirectionalamplification of these signals propagating in a common cable in oppositedirections.

To provide upstream communication capability to a multitude ofsubscribers, the upstream frequency channels are used in a so calledtime-division multiplexing (TDM) mode. Each cable modem is assigned atime slot, within which it is allowed to transmit information. The timeslots are assigned dynamically by a cable modem termination system(CMTS) disposed at the headend. The time slot information iscommunicated by CMTS to individual cable modems via an allocateddownstream channel. Subscribers access available network resources byusing a data communication bridge established between CMTS andindividual cable modems. Subscribers send data from their digitaldevices into cable modems, which then relay the data to the CMTS. TheCMTS, in turn, relays the information to appropriate services such asInternet servers, for example. Information destined to the subscriberdigital device is provided from the Internet servers to the CMTS, whichin turn relays the information to individual cable modems. The cablemodems then relay the information to the digital devices used by thesubscribers.

One popular communication standard for bidirectional data transport overa cable network is the Data Over Cable Service Interface Specification(DOCSIS). DOCSIS establishes rules of communication between CMTS andcable modems in a cable network. Three revisions currently exist for aNorth American DOCSIS standard, DOCSIS 1.x, 2.0, and 3.0. In addition tothe 6-MHz wide North American based DOCSIS standard, there exists aEuropean (Euro-DOCSIS) standard formatted for 8-MHz wide bandwidthchannels.

As cable communication systems grow and become more complex, the task ofproper system maintenance and troubleshooting becomes more difficult.The difficulty results from a random nature of signal bursts fromindividual cable modems. Although the cable modems are allocated timeslots in which they are allowed to transmit, the actual transmissiondepends on network activity of individual subscribers. Furthermore, theupstream signal bursts from the cable modems have a very short durationand arrive intermittently from a multitude of locations in the cablenetwork. Consequently, an upstream signal from a faulty location isinterspersed with upstream signals from locations that are functioningnormally. To be able to detect and eliminated faults in a cable network,it is important to identify faulty network locations.

While it is the headend where faulty signals' locations can be detected,it is the remote locations where the faults typically occur. Atechnician willing to fix a network problem must first analyze thesymptoms of the problem at the headend, then determine a geographicallocation of the fault, then drive there and attempt to fix the problem,then drive back to the headend and make sure the problem is fixed. Onecan employ two technicians equipped with a mobile communications deviceallowing them to communicate with each other, one technician remainingat the headend, and the second technician moving around in the field.This solution is costly because it increases labor costs. Furthermore,it is often difficult for the technician located at the headend toverbally describe the signal degradation patterns he observes to thetechnician located in the field.

The need to test the upstream signal path from a node disposed remotelyfrom a headend is recognized in the art. In U.S. Pat. No. 7,489,641 byMiller et al., upstream path test apparatus and method are described. Inthe method of Miller et al., test data packets are generated by a testdevice connected to a remote node. The test data packets have thedestination address of the test device itself. Therefore, when the testdata packets are transmitted to the headend, the headend automaticallyroutes them back to the test device. The test packets are then received,demodulated, and analyzed by the test device for faults.Disadvantageously, the test apparatus of Miller et al. cannotdistinguish whether the degradation has occurred in the upstream path orthe downstream path of the network.

In US Patent Application Publication 20050047442, Volpe et al. describesa test system that is configured to receive all upstream/downstreamchannels and demodulate upstream packets. A database of MAC/SIDaddresses is built, which allows the test system to eventually determinewhere the packets came from. Once the database is built, the origin offaulty data packets can be determined. Disadvantageously, the testsystem of Volpe lacks a capability to troubleshoot a particular upstreamsignal problem in real time.

Test systems for upstream signal analysis are known in the art. By wayof example, PathTrak™ and QAMTrack™ test systems, manufactured by JDSUCorporation located in San Jose, Calif., USA, allow for upstream signaldemodulation, analysis, and MAC address filtering at the headend. Theresults of the analysis can be made available for remote clients througha web browser interface. To test a particular node or a cable modem, onecan specify a MAC address of a cable modem under test. Provided thatenough time is given to the PathTrak system, an upstream signal burstfrom the cable modem under test can be captured at the headend andsubsequently analyzed for faults. Unfortunately, due to random nature ofupstream signal bursts, and due to long time required for packetsdemodulating and MAC filtering, analysis of faults at a particular nodecannot always be performed in an efficient way. Furthermore, even whenthe data packet is correctly identified, the PathTrak system does nothave an access to pre-equalization coefficients used by the cable modemunder test to generate the upstream burst. Without the pre-equalizationcoefficients, the test system cannot determine correctly the upstreampath signal distortions and the in-band spectral response.

The prior art is lacking a test method and a system allowing atechnician to analyze and troubleshoot upstream path problems byperforming a real-time analysis of test data packets generated by aspecific device in the field and received at the headend of a cablenetwork. The present invention provides such a device and a method.

SUMMARY OF THE INVENTION

A test system of the present invention performs capture and analysis ofupstream data packets generated by a specific terminal device of thecable network in process of a normal operation of the terminal device.The terminal device is specified by a technician using a test instrumentconnected to a node of the cable network. A test module, located at theheadend, performs the necessary analysis of the upstream data packetgenerated by the specified terminal device, and reports the results tothe test instrument. The test instrument itself can send upstream testdata packets to test the upstream path between the node and the headend.In one embodiment, the technician can request a particular terminaldevice to send, on a command from the test instrument relayed at theheadend to the terminal device, an upstream test packet of apre-determined length. In another embodiment, the test module is capableto pre-filter upstream data packets based on the packet length, to speedup upstream packet processing and analysis.

Besides packet length, the pre-filtering can also be based on the packetrepetition rate: only data packets of a pre-defined length or repetitionrate, or both, are selected for demodulation. The selected upstream datapackets are demodulated, and the origin of the data packets isdetermined by locating an identifier of a source of the data packet. Forexample, media access control (MAC) addresses can be used to determinethe packet origin. At least one quadrature amplitude modulation (QAM)quality parameter, such as modulation error ratio (MER), is collected,together with other upstream path quality parameters such as spectralresponse, ingress noise, etc. In one embodiment, pre-equalizationinformation of the upstream data packets can be taken into accountmathematically when calculating a spectral response of the upstream pathtraveled by the demodulated upstream data packets with matching MACaddresses.

In accordance with the invention there is provided a method of testingan upstream path of a cable network including a headend and a nodeconnected to the headend, the method comprising:

(a) sending from a test instrument operably connected to the node arequest to the headend to demodulate and obtain signal qualityinformation of an upstream data packet generated by a first terminaldevice connected to the cable network;

(b) receiving the test request at the headend;

(c) receiving and demodulating at the headend the upstream data packetgenerated by the first terminal device;

(d) obtaining, at the headend, at least one pre-equalization coefficientused by the first terminal device to generate the upstream data packet;

(e) obtaining, at the headend, the signal quality information of theupstream data packet demodulated in step (c), wherein the signal qualityinformation is corrected for pre-equalization using the at least onepre-equalization coefficient obtained in step (d).

In accordance with another aspect of the invention there is provided amethod of testing an upstream path of a cable network including aheadend and a node connected to the headend, the method comprising:

(a) sending from a test instrument operably connected to the node arequest to the headend to demodulate and obtain signal qualityinformation of an upstream data packet generated by a first terminaldevice connected to the cable network;

(b) receiving the test request at the headend;

(c) receiving and demodulating at the headend the upstream data packetgenerated by the first terminal device;

(d) obtaining, at the headend, the signal quality information of theupstream data packet demodulated in step (c),

wherein the upstream data packet has a target packet length, whereinstep (c) includes pre-filtering upstream data packets based on packetlength, so that only upstream data packets having the target packetlength are selected for demodulation.

In accordance with another aspect of the invention there is furtherprovided an apparatus for testing an upstream path of a cable networkincluding a headend and a node connected to the headend, the apparatuscomprising:

a test instrument for operably coupling to the node, wherein the testinstrument is configured for sending a request to the headend todemodulate and obtain signal quality information of an upstream datapacket generated by a first terminal device connected to the cablenetwork, and

a test module disposed at the headend, the test module including:

a communication circuit for receiving the test request and the upstreamdata packet;

a demodulator coupled to the communication circuit, for demodulating theupstream data packet generated by the first terminal device; and

a processor coupled to the demodulator and the communication circuit,for obtaining the signal quality information of the demodulated upstreamdata packet, wherein the signal quality information is corrected forpre-equalization using at least one pre-equalization coefficient used bythe first terminal device to generate the upstream data packet.

In one embodiment, the communication circuit is configured forcommunicating thus obtained signal quality information back to the testinstrument. The test instrument preferably has a display for displayingthe obtained signal quality information.

In accordance with another aspect of the invention, there is furtherprovided an apparatus for testing an upstream path of a cable networkincluding a headend and a node connected to the headend, the apparatuscomprising:

a test instrument for operably coupling to the node, wherein the testinstrument is configured for sending a request to the headend todemodulate and obtain signal quality information of an upstream datapacket generated by a first terminal device connected to the cablenetwork, and

a test module disposed at the headend, the test module including:

a communication circuit for receiving the test request and the upstreamdata packet;

a demodulator coupled to the communication circuit, for demodulating theupstream data packet generated by the first terminal device; and

a processor coupled to the demodulator and the communication circuit,for obtaining the signal quality information of the demodulated upstreamdata packet.

In the latter embodiment, the test instrument is configured to generateupstream data packets of a fixed length, and the test module isconfigured to pre-filter upstream data packets based on packet length,thus considerably reducing time of identifying the upstream data packetgenerated by the first terminal device.

In accordance with another aspect of the invention there is furtherprovided a cable network comprising the above test apparatus, theheadend, the node connected to the headend, and a plurality of terminaldevices connected to the node, the plurality of terminal devicesincluding a first terminal device,

wherein the first terminal device is configured for generation of anupstream test data packet of a first length selectable by the testmodule;

wherein the test module is configured to send, upon receiving a commandform the test instrument, a request to the first terminal device togenerate the upstream test data packet of the first length; and

wherein the test module is configured to pre-filter upstream datapackets based on packet length, for identifying the upstream test datapacket of the first length.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings in which:

FIG. 1 is s diagram of a cable network, showing a test instrument of theinvention coupled to a node of the network;

FIG. 2 is a block diagram of a test module located at the headend of thecable network of FIG. 1;

FIG. 3 is a block diagram of the test instrument of FIG. 1;

FIG. 4 is a block diagram showing a connection between the testinstrument, the test module of FIG. 2, and a cable modem terminationsystem (CMTS) of the cable network of FIG. 1;

FIG. 5 is a block diagram of a test system for testing the upstream pathaccording to an embodiment of the invention, showing flow of commandsbetween modules of the system;

FIG. 6 is a block diagram of upstream packet pre-filtering apparatusaccording to an embodiment of the invention;

FIG. 7 is an example view of a display of the test instrument, showingsignal quality information; and

FIG. 8 is a block diagram of method of obtaining the signal qualityinformation according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art.

Referring to FIG. 1, a cable network 100 includes a headend 101, aplurality of nodes 102, and a plurality of terminal devices 104. A cableplant 106 connects the terminal devices 104 to the respective nodes 102,and the nodes 102 to the headend 101. The terminal devices 104 caninclude digital TV boxes, VoIP phone systems, and cable modems. Inoperation, the headend sends downstream signals 108 to the terminaldevices 104 through the cable plant 106. The downstream signals 110include TV broadcasting signals, as well as DOCSIS downstream datapackets and control signals. The terminal devices 104 send upstreamsignals 110, for example DOCSIS upstream data bursts.

A test instrument 111 is operably coupled to one of the nodes 102. Thetest instrument 111 is configured to send a test request 112 to theheadend 101 to demodulate and obtain signal quality information of anupstream data packet 124 generated by a certain device connected to thecable network 100. This device can be one of the terminal devices 104,or the test instrument 111 itself. The device selected for testing ispreferably identified by its media access control (MAC) address. The MACaddress is selectable by an operator of the test instrument 111. In theembodiment shown in FIG. 1, it is the test instrument 111 that sends theupstream data packet 124, and, accordingly, the MAC address is that ofthe test instrument 111 itself. The test request 112 is received by atest module 121 located at the headend 101.

Referring to FIG. 2, the test module 121 includes a communicationcircuit 202 for receiving the test request 112 and the upstream datapacket 124, a demodulator 204 coupled to the communication circuit 202,for demodulating the upstream data packet 124, and a processor 206coupled to the demodulator 204 and to the communication circuit 202, forcalculating signal quality parameters of the demodulated upstream datapacket and for sending, through the communication circuit 202, signalquality information 128 back to the test instrument 111.

Herein, the term “signal quality information” means a quadratureamplitude modulation (QAM) quality parameter or information related toquality of an upstream signal. The signal quality information 128 mayinclude, by way of a non-limiting example, modulation error ratio (MER),in-band frequency response, in-band group delay, micro-reflections,impulse noise, a number of captured symbols in the upstream data packet,a number of erroneously detected symbols in the upstream data packet, aconstellation map, and an RF power level. The measured frequencyresponse may be corrected for pre-equalization used by the selectedterminal device to transmit the upstream data packet 124.Advantageously, combining QAM quality information with the frequencyresponse corrected for pre-equalization provides an operator of the testinstrument 111 with a broad, multi-level set of data sufficient fortroubleshooting most upstream path problems.

According to one embodiment of the invention, the pre-equalizationcoefficients are stored in a database 129 located at the headend 101.The database 129 associates the terminal devices 104 connected to thecable network 100 with pre-equalization coefficients that have been sentby the cable modem termination system (CMTS) of the headend 101 to theterminal devices 104 for use in generation of the upstream transmissionsignals 110. Upstream signal pre-equalization is known to a personskilled in the art to be a part of the DOCSIS communication protocol.

Referring now to FIG. 3, the test instrument 111 includes a packetgenerator 302 for generating the upstream data packets 124, apre-equalization circuit 304 coupled to the packet generator 302, forpre-equalizing the upstream data packets 124, a communication unit 306,coupled to the pre-equalization circuit 304, for communicatingpre-equalization coefficients to the test module 121 at the headend 101,and a display 308 for displaying the received pre-equalized signalquality information 128.

A technician wishing to test the upstream path at one of the nodes 102of the cable network 100, connects the test instrument 111 to theselected node 102 and causes the test instrument 111 to send the testrequest 112 to the test module 121. The test request 112 contains anidentifier, such as a MAC address, of a device that generates theupstream data packet 124 to be captured and demodulated by the testmodule 121. The technician can select a MAC address of one of theterminal devices 104 of the cable network 100, depending on which of thenodes 102 is being tested. In one embodiment, the terminal devices 104generate the upstream data packets 124 as a part of their normaloperation. In another embodiment, the terminal devices 104 areconfigured to send a “test” upstream data packet upon receiving arequest from the headend 101. The headend 101 generates this request inresponse to a command from the test instrument 111. Alternatively or inaddition, the technician can select the test instrument 111 itself to bethe source of the test upstream data packets. In this case, the upstreamdata packet 124 will be automatically generated by the test instrument111 shortly after issuing the test request 112.

The test request 112 is received by the test module 121. Upon receivingthe test request 112, the test module 121 starts capturing anddemodulating upstream data packets. Demodulated packets are screened fora device identifier contained in the test request 112. Upon finding thepacket with a matching device identifier, the processor 206 calculatesthe signal quality information 128, which can be correctedmathematically for pre-equalization used in the transmission of theupstream data packet 124. This is more beneficial than sending theupstream data packet 124 not pre-equalized, because the un-equalizedupstream data packet 124 may arrive too distorted for the demodulationto work, in which case no QAM quality information could be measured atall. As noted above, the pre-equalization coefficients can be obtainedfrom the database 129. In the embodiment where the test instrumentitself generates the upstream data packet, the pre-equalizationcoefficients can be supplied by the test instrument 111. For example,the pre-equalization coefficients can be included in the payload of theupstream data packet 124. After demodulation of the upstream data packet124, these coefficients will be obtained by the processor 206, which canuse them to correct the signal quality information 128 forpre-equalization.

In one embodiment, the obtained signal quality information 128 iscommunicated to the test instrument 111. The technician observes thetest results on the display 308 of the test instrument 111.Advantageously, this provides a real-time feedback for the technicianperforming repairs in the field. In another embodiment, the test module121 keeps performing tests and accumulating results at the headend, tobe observed by the technician at a later time at the headend orelsewhere in the field, using a Web browser interface connected to theInternet.

Referring now to FIG. 4, the test instrument 111 establishes a line ofcommunication 402 with the test module 121. The test module 121 performsmeasurements 406 of various signals in a CMTS 404, to obtain the signalquality data. The line of communication 402 can be formed based on aservice DOCSIS channel, an out-of-band (00B) service channel, or, iftime permits to make such a connection, a separate regular DOCSISbidirectional communications channel.

In FIG. 5, the above mentioned embodiment, in which the line ofcommunication 402 is formed using a regular DOCSIS bidirectionalcommunications channel, is illustrated in more detail. To perform signalquality measurements, the test instrument 111 first establishes aregular DOCSIS communication channel with the CMTS 404, as indicated by“Establish DOCSIS” command 504. Then, the test instrument 111 opens aTCP/IP communications session with an interoperations server 502, asindicated by “Open Interoperation Session” command 506. Theinteroperations server 502 is a Web based application that uses astandard Web browser to communicate with the test instrument 111. Theinteroperations server 502 provides the test instrument 111 with a listof currently active nodes 102 of the cable network 100. The user of thetest instrument 111 selects a node from the list, as shown at 508, and ameasurement session 510 is opened. At this point, the user can use thetest instrument 111 to send specific commands 512 to the test module 121to make measurements 515. Once all of the measurements 515 areperformed, the user closes the measurement session, as indicated at 514.Then, in response to a “Close Interoperation Session” command 516, theinteroperation session is closed.

According to one embodiment of the invention, pre-filtering of upstreamdata packets at the headend is used to improve speed and reliability ofdetecting and processing the upstream data packets 124 sent by the testinstrument 111. Referring now to FIG. 6, a pre-filtering apparatus 600is a part of the test module 121. The pre-filtering apparatus 600includes a packet duration filter 602, the demodulator 204, a MAC filter604, and the processor 206. In operation, the packet duration filter 602filters the upstream traffic 110, passing through packets having aduration of the upstream packet 124. The upstream packet 124 generatedby the test instrument 111 passes through the packet duration filter 602and is demodulated by the demodulator 204. The packet in digital form603 is filtered by the MAC address at the MAC filter 604. Signal qualityinformation 605 of the demodulated filtered packet 603 is received bythe processor 206, which calculates pre-equalized signal qualityinformation 607 based on at least one pre-equalization coefficient usedby the test instrument 111 to generate the upstream data packet 124.

Packet length pre-filtering can result in a dramatic improvement of thefiltering speed. For example, at an upstream packet rate of 1000 datapackets per second, plus 50 packets per second generated by the testinstrument 111, and at 100 millisecond demodulation time by thedemodulator 204 of the test module 121, the test module 121 will miss99% of data packets, so that only one test data packet 124 from the testinstrument 111 will be detected every two seconds. When the packetlength pre-filtering is implemented with 99% efficiency, approximatelyonly 10 packets out of the 1000 unwanted packets will be demodulated,which results in 60 packets per second traffic arriving at the input ofthe demodulator 204. Out of these 60 packets, 10 will be demodulatedevery second, five of six of these being the upstream data packets 124generated by the test instrument 111. Therefore, out of the 10demodulated packets per second, on average, approximately 8 will be theupstream data packets 124 generated by the test instrument 111.Therefore, 8 testing-useful packets will be detected every second, whichis 16 times improvement of the testing speed.

Turning now to FIG. 7, an example view 700 of the display 308 of thetest instrument 111 is presented. In FIG. 7, the MER and thepre-equalized MER (“UnEQ MER”) are equal to each other and are equal to20dB. The in-band response shows a spectral ripple 702.

Referring to FIG. 8, a method of testing the upstream path 110 of thecable network 100 is presented. At a step 802, the request 112 is sentfrom the test instrument 111 to the test module 121 of the headend 101to demodulate and obtain signal quality information of the upstream datapacket 124 generated by the test instrument 111. As noted above, any oneof the terminal devices 104 can also be selected at this step. Thedevice to receiving the packet 124 from is identified by a deviceidentifier selectable by the test instrument.

At a step 804, the test request 112 is received by the test module 121.At a step 806, the upstream data packet 124 generated by the testinstrument 111 or one of the terminal devices 104, as the case may be,is received and demodulated by the test module 121. At a step 808, atleast one pre-equalization coefficient used to transmit the upstreamdata packet 124 is obtained. At a step 810, the signal qualityinformation of the upstream data packet is obtained. At this step, thesignal quality information can be corrected for pre-equalization usingthe at least one pre-equalization coefficient obtained in the step 808.At an optional step 812, the pre-equalization corrected qualityinformation 128 is communicated to the test instrument 111. Finally, atan optional step 814 in FIG. 8, the obtained signal quality informationis displayed on the display 308 of the test instrument 111.

In one embodiment, in the step 808, the at least one pre-equalizationcoefficient is obtained from the terminal devices database 129. In anembodiment where it is the test instrument 111 that generates theupstream data packet 124, the at least one pre-equalization coefficientis communicated by the test instrument 111 to the test module 121.Preferably, the upstream data packet 124 includes the at least onepre-equalization coefficient digitally encoded therein, so that once thepacket 124 is received and demodulated, the pre-equalization informationis immediately available to the test module 121 for mathematicalcorrection of the measured in-band spectral response.

Once the upstream data packet 124 is demodulated by the test module 121,QAM quality information can be included in the signal qualityinformation 128. The signal quality information 128 may be communicatedto the test instrument 111 in a variety of ways, for example by using adedicated DOCSIS downstream channel, or by using a DOCSIS servicechannel. The step 806 of receiving and demodulating the upstream datapacket 124 preferably includes a step of pre-filtering upstream datapackets based on the packet length, as explained above, so that onlyupstream data packets having the target packet length are selected forthe time-consuming step of demodulation. In one embodiment, the targetpacket length is selected by obtaining a probability distribution ofupstream packet lengths in the cable network, and selecting a packetlength having a probability of no more than a certain value, preferably25%, of a maximum probability of the probability distribution, as thetarget packet length. The target packet length has to be selected out ofthe set of lengths allowed by the CMTS 404 according to DOCSIScommunications protocol.

In one embodiment, the test instrument is configured for generating theupstream data packets 124 of a target packet length periodically, thatis, at regular time intervals. The upstream traffic 110 is filteredbased on arrival time of the upstream data packets of the target packetlength, thereby identifying the upstream data packets generated by thetest instrument at the regular time intervals.

Advantageously, the functionality of upstream packet pre-filtering basedon the arrival time or frequency of the upstream data packets 124generated by the test instruments 111 can be used to automaticallydiscover and register the test instrument 111 at the headend 101 of thecable network 100. The test instrument 111 sends a command to the testmodule 121 to pre-filter upstream data packets based on the packetlength and arrival time (or frequency). This pre-filtering is performedbefore demodulation and thus can be done quickly and efficiently. Thepre-filtered upstream data packets 124 are analyzed for a device ID. Ifall of them have the same device ID, it is taken to be the ID of thetest instrument.

1-20. (canceled)
 21. A method of testing an upstream path of a cablenetwork, the method comprising: receiving, at a headend, an upstreamdata packet generated by a terminal device connected to the cablenetwork, wherein the upstream data packet includes a test packet of aselected target packet length; filtering, by a packet duration filter atthe headend, the received upstream data packet and passing, though thefilter, received data packets determined to have a length equal to thetarget packet length; demodulating the filtered received data packetsdetermined to have a length equal to the target packet length andscanning the demodulated data packets to identify a test request and adevice identifier identifying the connected terminal device;determining, at the headend, signal quality information for the terminaldevice based on the demodulated test packet; and communicating thesignal quality information to a test instrument.
 22. The method of claim21, wherein the target packet length is selected by: obtaining aprobability distribution of upstream packet lengths in the cablenetwork; and selecting, as the target packet length, a packet lengthhaving a probability of no more than 25% of a maximum probability of theprobability distribution.
 23. The method of claim 21, wherein thereceiving at a headend of an upstream data packet generated by theterminal device includes receiving, at a regular time interval, upstreamdata packets of the target packet length from the terminal device,wherein the filtering at the headend includes filtering upstream datapackets based on arrival time of upstream data packets of the targetpacket length, and identifying, based on the filtering of the upstreamdata packets, test packets generated by the terminal device at theregular time intervals.
 24. The method of claim 21, comprising:transmitting a request for a test packet to the terminal device.
 25. Themethod of claim 24, wherein transmitting the request for the test packetincludes: receiving a test request message from the test instrument,wherein the test request message includes a terminal device identifier.26. The method of claim 21, wherein determining signal qualityinformation for the terminal device includes: correcting the determinedsignal quality information to account for pre-equalization of theupstream data packet, wherein the corrected signal quality informationincludes at least one of modulation error ratio (MER), in-band frequencyresponse, in-band group delay, micro-reflections, impulse noise, numberof captured symbols in the upstream data packet, number of erroneouslydetected symbols in the upstream data packet, a constellation map, andRF power level.
 27. The method of claim 26, wherein correcting thedetermined signal quality includes: correcting the determined signalquality using at least one upstream pre-equalization coefficient used bythe terminal device to generate the upstream data packet.
 28. The methodof claim 27, wherein the at least one pre-equalization coefficient isreceived by the headend from the terminal device.
 29. The method ofclaim 21, wherein the terminal device is the test instrument.
 30. Amethod of testing an upstream path of a cable network, the methodcomprising: receiving, at a headend, an upstream data packet generatedby a terminal device connected to the cable network, wherein theupstream data packet includes a test packet; filtering, by a packetduration filter at the headend, the received upstream data packet andpassing the test packet though the packet duration filter; demodulatingthe test packet and scanning the demodulated test packet to identify atest request and a device identifier identifying the terminal device;determining, at the headend, signal quality information for the terminaldevice based on the demodulated test packet; receiving, from theterminal device, at least one upstream pre-equalization coefficient usedby the terminal device to generate the upstream data packet; andcorrecting the determined signal quality information to account forpre-equalization of the upstream data packet using the received at leastone upstream pre-equalization coefficient.
 31. The method of claim 30,wherein the test packet has a target packet length, and wherein thepassing of the test packet though the packet duration filter comprisespassing an upstream data packet having the target packet length throughthe packet duration filter.
 32. A system for testing an upstream path ofa cable network, the system comprising: a test module disposed at aheadend of the cable network, the test module including: a communicationcircuit for receiving: an upstream data packet generated by a terminaldevice connected to a node of the cable network, wherein the upstreamdata packet includes a test packet of a target packet length; and a testrequest message to obtain signal quality information of the upstreamdata packet received from the terminal device; a demodulator coupled tothe communication circuit to demodulate the upstream data packetgenerated by the terminal device; a packet duration filter to filter thereceived upstream data packet and pass though the filter received datapackets determined to have a length equal to the target packet length;and a processor, coupled to the demodulator and the communicationcircuit, to determine the signal quality information of the demodulatedupstream data packet,
 33. The system of claim 32, further comprising atest instrument, the test instrument comprising: a packet generator togenerate the upstream data packet including the test packet of thetarget packet length; a pre-equalization circuit coupled to the packetgenerator to pre-equalize the upstream data packet; a communicationcircuit to communicate pre-equalization coefficients to the test module;and a display to display the received pre-equalized signal qualityinformation.
 34. The test instrument of claim 33, wherein the packetgenerator is to generate, at regular time intervals, the upstream datapacket of the target packet length, wherein the test module is to filterthe upstream data packets based on arrival time of the upstream datapackets of the target packet length, thereby identifying the upstreamdata packets generated by the test instrument at the regular timeintervals.
 35. An apparatus for testing an upstream path of a cablenetwork, the apparatus comprising: a packet generator to generate, atregular time intervals, an upstream data packet including a test packetof a defined packet length; a pre-equalization circuit coupled to thepacket generator to pre-equalize the upstream data packet; acommunication circuit to communicate pre-equalization coefficients to aheadend of the cable network, and to receive pre-equalized signalquality information from the headend; and a display to display thereceived pre-equalized signal quality information.
 36. The apparatus ofclaim 35, wherein the apparatus communicates with the headend via adedicated Data Over Cable Service Interface Specification (DOC SIS)connection.
 37. The apparatus of claim 35, wherein the communicationcircuit is to transmit a test request message to the headend.